The prior art uses optical cavities to filter electromagnetic radiation. Optical cavities are also used in conjunction with optical amplifiers to generate coherent radiation. More specifically, ring-type optical cavity filters can filter and generate radiation whose instantaneous frequency is changing in time. Changing the optical path length of the cavity causes the resonant modes of the cavity to shift in frequency at the same time rate as the desired frequency shift rate of the radiation to be filtered or generated. The optical path length of a cavity is changed with electro-optic devices such as rotating brewster plates as in the Model #699-05 ring cavity laser produced by Coherent Corp. or by electro-optic crystal devices such as the Series #620 phase/frequency modulators produced by Interactive Radiation, Inc. in Northvale, N.J. The optical path length of a cavity may also be changed by mechanically or piezoelectrically moving one of the mirrors which form the cavity. Electro-optic devices are currently not capable of causing path length changes of more than about 0.01% in a typical cavity. The mechanical and piezoelectric means are difficult to construct with sufficient precision for causing a cavity path length change corresponding to a frequency shift of more than approximately 100 resonant mode spacings for a one-meter long cavity. Further, shifting the frequency of the resonant modes of a cavity by changing the cavity length also changes the difference in frequency between two different modes.